A transconductor is basically a voltage-controlled differential stage with transconductance. It is used to provide integrators incorporated in active filters but also to provide oscillators and circuits for the conversion of impedances.
In recent years there have been proposed various techniques for providing time-continuous integrated filters. A technique which has gained acceptance as the most effective for high frequency applications is one using a BiCMOS transconductor stage as a basic block of the filter.
Inside a BiCMOS transconductor filter an open-loop voltage-current conversion is performed, and a so-called non-dominant pole of the filter is not limited by the unit frequency gain of an operational amplifier circuit portion of the filter. This is an advantage over filters provided in conformity with CMOS technology.
However, transconductor filters also have a shortcoming of being particularly sensitive to parasitic capacitors as a result of their open-loop operating mode. This limits the amplitude of their pass band, particularly at high frequencies.
A transconductor filter which seeks to obviate this limitation in the pass band is disclosed in U.S. Pat. No. 5,332,937.
The solution disclosed in this U.S. patent, although advantageous in some ways, does not allow band amplitudes capable of meeting current requirements.
In some applications linked to a reading of information in computer mass memories, e.g., in the reading of a hard disc drive, there is a need to further extend a pass band of a signal to be filtered.
A filter with an improved frequency performance is disclosed in the article "Tunable BiCMOS Continuous-Time Filter for High Frequency Application," IEEE Journal of Solid State Circuits. December 1992, page 1905. The article proposes varying a transconductance gm of a transconductor filter depending on the requirements of the field of application. The article considers variations in the transconductance gm between one and four.
An analysis made in this article shows that there exists an upper limit to the amplitude of an input signal applicable to a transconductor stage of the filter. Beyond this limit the linearity characteristics of the stage are seriously impaired.
The solution disclosed in the above mentioned article does not meet current application requirements.